1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to an array panel for a transflective liquid crystal display device.
2. Discussion of the Related Art
In general, the LCD device includes two substrates, which are spaced apart and facing each other, and a liquid crystal layer interposed between the two substrates. Each of the substrates includes an electrode and the electrodes of each substrate are facing each other, also. Voltage is applied to each electrode and an electric field is induced between the electrodes. An alignment of the liquid crystal molecule is changed by the intensity of the electric field, and the LCD device displays a picture by transmissivity of the light varying according to the arrangement of the liquid crystal molecules.
Because the liquid crystal display device is not luminescent, it needs an additional light source in order to display images, and the liquid crystal display device is categorized into a transmissive type and a reflective type depending on the kind of light source.
In the transmissive type, a back light behind a liquid crystal panel is used as a light source. Light incident from the back light penetrates the liquid crystal panel, and the amount of the transmitted light is controlled according to the alignment of the liquid crystal molecules. Here, the substrates must be transparent and the electrode of each substrate must also be formed of transparent conductive material. As the transmissive liquid crystal display device uses the back light as a light source, it can display a bright image in dark surroundings. By the way, because an amount of the transmitted light is very small for the light incident from the back light, the brightness of the back light should be increased in order to increase the brightness of the LCD device. Consequently, the transmissive liquid crystal display device has high power consumption due to the back light.
On the other hand, in the reflective type LCD device, sunlight or artificial light is used as a light source of the LCD device. The light incident from the outside is reflected at a reflective plate of the LCD device according to the arrangement of the liquid crystal molecules. Since there is no back light, the reflective type LCD device has much lower power consumption than the transmissive type LCD device. However, the reflective type LCD device cannot be used in dark places because it depends on an external light source.
Therefore, a transflective LCD device, which can be used both in a transmissive mode and in a reflective mode, has been recently proposed. A conventional transflective LCD device will be described hereinafter more in detail.
FIG. 1 is a cross-sectional view of a conventional transflective LCD device. In FIG. 1, the conventional transflective LCD device has a lower substrate 10 and an upper substrate 30, and the substrates 10 and 30 are spaced apart from and facing each other.
A pixel electrode 20 is formed on the inner surface of the lower substrate 10 and connected to the thin film transistor (not shown) formed on the inner surface of the lower substrate 10. The pixel electrode 20 includes a transmissive electrode 21 and a reflective electrode 22. The reflective electrode 22 has a hole in which the transmissive electrode 21 is located. The transmissive electrode 21 is formed of a transparent conductive material such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO), which has high transmittance. The reflective electrode 22 is formed of an opaque conductive material such as aluminum (Al), which has high reflectance and low resistivity.
Next, a color filter 40, which corresponds to the pixel electrode 20, is formed on the inner surface of the upper substrate 30 and a common electrode 50 is formed on the color filter 40. The common electrode 50 is also made of the transparent conductive material.
A liquid crystal layer 60 is disposed between the lower and upper substrates 10 and 30, and molecules of the liquid crystal layer 60 are arranged horizontally with respect to the substrates 10 and 30.
On the outer surfaces of the substrates 10 and 30, a first retardation film 71 and a second retardation film 72 are arranged, respectively. The first and second retardation films 71 and 72 change the polarized state of light. In case of the first and second retardation films 71 and 72 having a phase difference of λ/4 (λ=550 nm), an incident circular polarized light changes into linear polarized light, and an incident linear polarized light changes into circular polarized light.
A first polarizer 81 and a second polarizer 82 are arranged on the outer surface of the first and second retardation films 71 and 72. The second polarizer 82 is an analyzer, and the transmission axis of the second polarizer 82 has an angle of 90 degrees with the transmission axis of the first polarizer 81.
Next, a back light 90 is located under the outside of the first polarizer 81. The back light 90 is used as a light source of a tranmissive mode of the transflective LCD device.
This transflective LCD device is a normally white mode and in such case, white light is emitted when voltage is not applied. By the way, the transflective LCD device is planned on the basis of the reflective mode. Accordingly, transmittance of the transmissive mode becomes only 50% of the transmittance of the reflective mode when the voltage is not applied, and thus gray light is emitted in the transmissive mode.
FIG. 2 illustrates the transflective LCD device to solve the above problem. In FIG. 2, the transflective LCD device is divided into a transmissive region “A” and a reflective region “B”.
The transflective LCD device has a lower substrate 110 and an upper substrate 160 facing apart from each other. A first passivation layer 120 is formed on the inner surface of the lower substrate 110, and the first passivation layer 120 has a first trasmissive hole 122 in the transmissive region “A”. A transmissive electrode 130 of a transparent conductive material is formed on the first passivation layer 120. Next, a second passivation layer 140 is formed on the transmissive electrode 130, and a reflective electrode 150 is formed on the second passivation layer 140. The reflective electrode 150 has a second transmissive hole 152 exposing the transmissive electrode 130 on the first transmissive hole 122. On the other hand, a thin film transistor (not shown) is formed on the inner surface of the lower substrate 110, and the thin film transistor is connected electrically to not only the transmissive electrode 130 but also the reflective electrode 150.
A color filter 161 is formed on the inner surface of the upper substrate 160 and a common electrode 162 is formed on the color filter 161.
Next, retardation films 171 and 172 are arranged on the outer surface of the lower and upper substrates 110 and 160, respectively. Polarizers 181 and 182 are arranged on the outer surface of the respective retardation film 171 and 172. A back light 190 is located under the lower polarizer 181.
A liquid crystal layer 200 is disposed between the reflective electrode 150 and the common electrode 162. The liquid crystal molecules of the liquid crystal layer 200 are arranged horizontally with respect to the substrates 110 and 160. The liquid crystal layer 200 has a positive permittivity anisotropy value, so the liquid crystal molecules are arranged parallel to a direction of the electric field induced between the reflective electrode 150 and the common electrode 162 when voltage is applied to the electrodes 130, 150 and 162.
A phase difference of the liquid crystal layer depends on the refractive index anisotropy value (Δn) and the thickness (d) of the liquid crystal layer. Therefore, the phase difference of the liquid crystal layer can be controlled by changing the thickness of the liquid crystal layer.
Accordingly, as shown in FIG. 2, the first passivation layer 120 has a first transmissive hole 122 so that the brightness in the transmissive mode and the reflective mode may be made uniform. At this time, it is desirable that the liquid crystal layer 200 in the transmissive region “A” has twice thickness of the liquid crystal layer 200 in the reflective region “B”.
The polarized situations of the transflective LCD device of FIG. 2 are illustrated in FIGS. 3A and 3B and in FIGS. 4A and 4B.
FIGS. 3A and 3B shows the polarized states before and after applying voltage in the reflective mode, respectively. Here, the polarized situations are represented according to a progressing direction of the light. Meanwhile, y-axis is a direction parallel to the substrates 110 and 160 of FIG. 2 and z-axis is a direction perpendicular to the substrates 110 and 160. And x-axis is defined in a direction perpendicular to both the y-axis and the z-axis. Therefore, the transmission axis of the upper polarizer 182 has an angle of 135 degrees to the x-axis and the transmission axis of the lower polarizer 181 has an angle of 45 degrees to the x-axis as watched from the bottom of the liquid crystal panel. In the meantime, the transmission axis of the upper polarizer 182 has an angle of 45 degrees to the x-axis as watched from the top of the liquid crystal panel.
At this time, the liquid crystal layer 200 disposed in the reflective region “B” of FIG. 2 has a phase difference of λ/4 and is right-circulary polarized before applying voltage.
The optical axis of the lower retardation film 171 of FIG. 2 is parallel to the y-axis and the lower retardation film 171 is right-handed. Therefore, an incident light of 45 degrees is right-circularly polarized and an incident light to be right-circularly polarized is linearly polarized at an angle of 135 degrees. An incident light of 135 degrees is left-circularly polarized and an incident light to be left-circularly polarized is linearly polarized at an angle of 45 degrees.
On the other hand, the optical axis of the upper retardation film 172 of FIG. 2 is parallel to the x-axis and the upper retardation film 172 is left-handed. And thus an incident light of 45 degrees to the x-axis is left-circularly polarized and an incident light to be left-circularly polarized is linearly polarized at an angle of 135 degrees. Next, an incident light of 135 degrees to the x-axis is right-circularly polarized and an incident light to be right-circularly polarized is linearly polarized at an angle of 45 degrees.
In FIG. 3A, as voltage is not applied to the transflective LCD device, a light is linearly polarized an angle of 45 degrees to the x-axis through the upper polarizer 182 of FIG. 2, and the linearly polarized light is left-circularly polarized through the upper retardation film 172. Next, the left-circularly polarized light goes through the liquid crystal layer 200 disposed in the reflective region “B” to be linearly polarized at an angle of 45 degrees from the left-circularly polarization. This linearly polarized light is reflected at the reflective electrode 150 of FIG. 2, and so the progressing direction of the light changes. Accordingly, the reflective light has a polarizing angle of 135 degrees to the x-axis. Next, the linearly polarized light at an angle of 135 degrees is left-circularly polarized through the liquid crystal layer 200 disposed in the reflective region “B”. The left-circularly polarized light is linearly polarized at an angle of 135 degrees again through the upper retardation film 172. As the linearly polarized light has a polarizing direction to coincide with the transmission axis of the upper polarizer 182, the linearly polarized light is all transmitted. Therefore, the displayed picture becomes white.
Next, in FIG. 3B, when voltage is applied to the transflective LCD device, a light is linearly polarized at an angle of 45 degrees to the x-axis through the upper polarizer 182 of FIG. 2, and the linearly polarized light is left-circularly polarized through the upper retardation film 172. The left-circularly polarized light goes through the liquid crystal layer 200 disposed in the reflective region “B” without change of the polarized state. Next, the left-circularly polarized light is reflected at the reflective electrode 150 of FIG. 2, and so the left-circularly polarized light is right-circularly polarized. The right-circularly polarized light does not change through the liquid crystal layer 200 disposed in the reflective region “B”. Next, the right-circularly polarized light is linearly polarized at an angle of 45 degrees through the upper retardation film 172, and the linearly polarized light has a direction perpendicular to the transmission axis of the upper polarizer 182. Accordingly, the linearly polarized light is not transmitted, and thus the displayed picture becomes black.
FIGS. 4A and 4B shows the polarized states before and after applying voltage in the transmissive mode, respectively. At this time, the liquid crystal layer 200 disposed in the transmissive region “A” of FIG. 2 has a phase difference of λ/2 before applying voltage.
In FIG. 4A, as voltage is not applied to the transflective LCD device, light incident from the back light 190 to the lower polarizer 181 of FIG. 2 is linearly polarized an angle of 45 degrees to the x-axis through the lower polarizer 181, and the linearly polarized light is right-circularly polarized through the lower retardation film 171. Next, the right-circularly polarized light goes through the transmissive electrode 130 and the right-circularly polarized light is left-circularly polarized through the liquid crystal layer 200 disposed in the transmissive region “A”. The left-circularly polarized light is linearly polarized at an angle of 135 degrees through the upper retardation film 172. As the linearly polarized light has a polarizing direction to coincide with the transmission axis of the upper polarizer 182, the linearly polarized light is all transmitted. Therefore, the displayed picture becomes white.
On the other hand, in FIG. 4B, when voltage is applied to the transflective LCD device, light from the back light 190 is linearly polarized an angle of 45 degrees to the x-axis through the lower polarizer 181 of FIG. 2, and the linearly polarized light is right-circularly polarized through the lower retardation film 171. The right-circularly polarized light goes through the transmissive electrode and the liquid crystal layer 200 disposed in the transmissive region “A” without change of the polarized state. Next, the right-circularly polarized light is linearly polarized at an angle of 45 degrees through the upper retardation film 172, and the linearly polarized light has a direction to be perpendicular to the transmission axis of the upper polarizer 182. Accordingly, the linearly polarized light is not transmitted, and thus the displayed picture becomes black.
As stated above, different thickness between the transmissive region and the reflective region makes the displayed picture uniform and substantially dark in the black mode. Therefore, the contrast ratio of the transflective LCD device is increased and the quality of picture improves.
By the way, in the case of forming the transmissive hole 122 of FIG. 2, an inclined portion is formed between the transmissive region “A” and the reflective region “B”, and the thickness of the liquid crystal layer 200 disposed in the inclined portion changes continuously. Accordingly, when the voltage is applied to the transflective LCD device, a fringe field is produced in the inclined portion and a distortion occurs. Also, the phase difference of the liquid crystal layer varies in the region, and thus light leakage occurs.
The structure of the array panel for the transflective LCD device to prevent the leakage light like this is suggested in the Japanese publication No. 2000-275660. FIG. 5 is the representative drawing of 2000-275660.
As shown in FIG. 5, a thin film transistor, which includes a double-layered gate electrode 8 and 9, an active layer 11, ohmic contact layers 12, a source electrode 14 and a drain electrode 15, is formed on a substrate 1. A transparent electrode 13, which connects to the thin film transistor, is formed on the substrate 1. An interlayer 3 of a photosensitive resin is formed on the thin film transistor and the transparent electrode 13. The interlayer 13 has a transmissive hole exposing a part of the transparent electrode 13 and inclined portion 17. The inclined portion 17 is a border area between a transmissive region and a reflective region. A reflector 4 or 5 is formed on the interlayer 3, and the reflector 4 or 5 covers a part of the inclined portion 17. A concave-convex section 18 is formed between the interlayer 3 and the reflector 4 or 5 in order to increase reflection. In the array panel of 2000-275660, the effective area of the transmissive mode and the reflective mode has to be equal in order to get a stable picture. Here, the inclined portion 17 does not belong to the transmissive mode or the reflective mode. Therefore, the inclined portion is neither the transmissive region nor the reflective region. If one end of the reflector 4 or 5 is located on the inclined portion 17, the effective area of the transmissive region and the reflective region is not influenced even though there are a bit errors.
However, as stated above, the leakage light occurs in the inclined area 17. If the reflector 4 or 5 is formed covering all the inclined portion 17 in order to prevent the light leakage, the reflector 4 or 5 contacts the transparent electrode 13, and thus galvanic corrosion happens between the transparent electrode 13 and the reflector 4 or 5. Accordingly, the reflector 4 or 5 should not contact the transparent electrode 13 and must cover only a part of the inclined portion 17.